What is the reason for the capacity decline of spinel lithium manganese oxide?

The biggest disadvantage of spinel lithium manganese-oxygen solid solution as a cathode material for lithium ion secondary batteries is that the capacity decay is more serious. The reason for the capacity decay is all from the change of spinel structure, which can be summarized as the following aspects.

Lithium manganese oxide for lithium ion batteries

(1) Jahn-Teller effect and formation of passivation layer
The Jahn-Teller effect that occurs during the charging and discharging process of spinel LiMn₂O₄ leads to the distortion of the spinel lattice and is accompanied by a large volume change. The c/a ratio of the structure increased by 16%, which resulted in the collapse of the spinel structure. Especially near the end of the 4V discharge platform, the spinel lithium manganese oxide particles on the surface are over-discharged into Li₂Mn₂O₄. Due to the incompatibility between the tetragonal crystal system on the surface and the cubic crystal system inside the particles, the structural integrity and integrity are seriously damaged. The effective contact between the particles affects the diffusion of lithium ions and the electrical conductivity between the particles, resulting in a loss of capacity.

LiMn2O4  Jahn-Teller effect

The capacity loss during the cycle is the result of the synergistic effect of the physical effects caused by the distortion of Jahn-Teller and the chemical effects of high-temperature storage. In the case of medium-rate discharge, there may be a local concentration gradient of Li⁺ on the surface of the active material ions, leading to the formation of the Jahn-Teller twisted phase Li₂Mn₂O₄. The mismatch of lattice constants causes surface fracture and pulverization. The increase of specific surface and the formation of Mn³⁺ rich phase Li₂Mn₂O₄ are conducive to disproportionation and dissolution of Mn³⁺. The formation of a passivation layer with relatively low ion and electronic conductivity on the ion surface is one of the reasons for the capacity decline of LiMn₂O₄. The ion exchange reaction advances from the surface of the active material ion to its core, and the new phase formed covers the original active material, so that a passivation layer with low ion and electronic conductivity is formed on the surface of the ion. This passivation layer is composed of lithium and Manganese is a water-soluble substance, and thickens as the number of cycles increases. The dissolution of manganese and the decomposition of the electrolyte lead to the formation of a passivation layer, and high temperature conditions are more conducive to the progress of these reactions. This will increase the contact resistance between the active material particles and the Li⁺ migration resistance, which will increase the polarization of the battery, incomplete discharge capacity, and decrease the capacity.

(2) Dissolution of manganese destroys the spinel structure
The dissolution of spinel LiMn₂O₄ in the electrolyte is the main reason for the capacity decline of spinel LiMn₂O₄. The direct cause of the dissolution of spinel LiMn₂O₄ during the cycle is the disproportionation reaction of Mn³⁺:
H₂O＋LiPF₆→POF₃+2HF+LiF
4H⁺+2LiMn³⁺Mn⁴⁺O₄→3λ-MnO₂+Mn²⁺+2Li⁺+2H₂O

Dissolution of manganese destroys the spinel structure

Due to the presence of a small amount of water in the electrolyte, it reacts with LiPF₆ in the electrolyte to generate HF acid, which leads to the disproportionation reaction of spinel LiMn₂O₄, dissolving Mn²⁺ into the electrolyte, and destroying the spinel structure. The dissolution of Mn³⁺ is also the root cause of the Jahn-Teller effect. When n(Mn³⁺)/n(Mn⁴⁺) is greater than 1, the spinel crystal lattice will transform from the trigonal phase to the tetragonal phase, resulting in a larger crystal lattice volume of the material. Change, causing the crystal lattice to be destroyed or even collapsed. In addition, the accumulative oxidative decomposition of the electrolyte will inevitably lead to deterioration of electrolyte performance, increase in ohmic polarization, and decrease in battery performance.

(3) The electrolyte is decomposed at high potential and the spinel structure is destroyed
Most of the electrolyte solvents used in lithium ion batteries are organic carbonates, such as PC, EC, BC, DMC, DEC, EMC, DME or their mixtures. During the charge-discharge cycle, these solvents will undergo decomposition reactions, and their decomposition products may form Li₂CO₃ film on the surface of the active material, which will increase the polarization of the battery and cause the spinel LiMn₂O₄ and the capacity of the positive electrode material to decrease during the cycle. When charged to a high voltage, the catalysis of λ-MnO₂ decomposes the organic electrolyte, and then λ-MnO₂ is reduced to MnO, which eventually leads to the dissolution of Mn and the destruction of the spinel structure. The decomposition of the electrolyte is not only related to the conductive agent and the state of charge and discharge, but also closely related to the temperature. As the temperature increases, the decomposition of the electrolyte intensifies.

Electrolyte destroys spinel structure

Under high temperature conditions, the capacity loss mainly occurs in the high voltage region (4.1V). With the dissolution of manganese in the material, the two-phase structure in this region gradually becomes a stable single-phase structure (LiMn_2-xO_4-x), and at the same time the entire Direct dissolution of Mn₂O₃ occurs in the 4V zone, and this structural change constitutes the main part of the capacity loss. LiMnO₃ and Li₂Mn₄O₉, the products of manganese dissolution, have no electrochemical activity in the 4V region. In addition, the disproportionation and dissolution of Mn³⁺ form a cation-deficient spinel phase, which damages the crystal lattice and blocks Li⁺ diffusion channels. Regardless of the changes in the structure, the dissolution of manganese causes an increase in resistance when the active material ions are in contact with each other. Under high temperature conditions, the rate of electrolyte oxidation and decomposition speeds up, the change of manganese concentration also speeds up, and the capacity loss becomes more serious.

The specific surface of spinel has a great influence on the dissolution rate of manganese. Therefore, the easiest way to improve high-temperature performance is to reduce the specific surface of the material, thereby reducing the contact area between the electrode material and the electrolyte. Although this can improve the high-temperature performance of the spinel to a certain extent, the large particle size may cause difficulty in lithium ion diffusion, reduce the rate characteristics and discharge capacity of the battery, and the processability of the material will be worse, which will easily cause the diaphragm to perforate. More practical and feasible methods need to be explored. For example, the surface treatment of Li_1.05 Mn_1.95O₄, the use of lithium boron oxide (LBO) glass phase to coat the cubic spinel, reduce the specific surface of the material to slow down the erosion of hydrogen fluoride; treat the spinel with acetylacetone Stone, the agent complexes with manganese on the surface of the spinel to inhibit the dissolution of manganese. These two methods can also improve the high-temperature performance of spinel; a metal carbonate passivation layer with a mass fraction of 0.4% to 1.5% is prepared on the surface of lithium manganese oxide. The oxygen-enriched spinel Li_1+xMn₂O₄ is first coated with LiOH, and then treated at high temperature in CO2 to form a Li₂CO₃ coating layer on the surface of the material, thereby improving the high-temperature storage performance of the material under charge and discharge, and the specific capacity has seen an increase. In comparison, coating with LiOH will reduce its capacity due to the diffusion of Li⁺ into the material to form Li_1+xMn₂O₄ when processed in air. In addition, coating with cobalt acetate and CO₂ treatment can effectively reduce the irreversible capacity loss during storage of the 60℃ discharge platform.